Breakwaters are generally shore-parallel structures that reduce the amount of wave energy reaching the protected area. They are similar to natural bars, reefs or near shore islands and are designed to dissipate wave energy. For breakwaters protecting harbors, the breakwater acts as a barrier to wave energy and often to direct alongshore sediment transport away from the harbor. For shore protection, offshore breakwaters provide a reduction in wave energy in the lee of the structure slowing the littoral drift, producing sediment deposition and a shoreline bulge or “salient” feature in the sheltered area behind the breakwater. Some alongshore sediment transport may continue along the coast behind a near shore breakwater.
There are various types of breakwaters. These include:
Headland breakwaters, a series of breakwaters constructed in an “attached” fashion to the shoreline and angled in the direction of predominant wave approach such that the shoreline behind the features evolves into a natural “crenulate” or log spiral embayment.
Detached breakwaters that are constructed away from the shoreline, usually a slight distance offshore. They are detached from the shoreline, and are designed to promote beach deposition on their leeside.
Single breakwaters that may be attached or detached depending on what they are being designed to protect. A single detached breakwater may protect a small section of shoreline. A single attached breakwater, may be a long structure designed to shelter marinas or harbors from wave action.
System breakwaters refer to two or more detached, offshore breakwaters constructed along an extensive length of shoreline.
Rubble mound jetties are often referred to as breakwaters. They are oriented shore-perpendicular and usually built as a pair at a natural inlet, to provide extension of a navigation channel some distance from the natural shoreline. These structures redirect the sediment transport away from the navigation channel and constrain the tidal flow in the channel in order to make an efficient channel that requires little maintenance for navigation compared to a natural inlet.
Breakwaters are typically constructed in high wave energy environments using large armor stone, or pre-cast concrete units or blocks. In lower wave-energy environments, grout-filled fabric bags, gabions and other proprietary units have been utilized. Typical breakwater design is similar to that of a revetment, with a core or filter layer of smaller stone, overlain by the armoring layer of armor stone or pre-cast concrete units.
Armor units conventionally constructed of concrete are typically used to protect rubble mound structures in relatively high wave environments or where stone armor is not readily available. Rubble mound structures include breakwaters, revetments, jetties, caissons, groins and the like. Coastal rubble mounds are gravity structures. Conventional armor units are heavy in order to prevent displacement or rocking from waves and currents.
Armor units are typically displaced by one or both of two dominant modes of structure failure. The first is displacement of the armor which leads to exposure and erosion of filter layers and subsequently the core. The second is armor breakage. The breakwater or revetment capacity will be significantly reduced if either of these two failure modes occurs and progressive failure of the structure made much more likely. The under layer (filter layer) is sized so as to not move under undamaged armor and to prevent interior stone (e.g., small quarry-run stone) from escaping.
A wave is described by its height, length, and the nature of breaking. The wave height is the dominant forcing parameter considered in designing armor units. Other parameters include wave length, water depth, structure shape and height, armor layer porosity, degree of armor interlocking, inter-unit friction, and armor density relative to the water.
It is known that waves exert forces on armor units in all directions. Slender armor units usually require steel reinforcement while more stout armor shapes do not. Adequate steel (rebar) reinforcement increases material costs by roughly 100% over un-reinforced concrete. Both steel and polypropylene fiber reinforcement have been used to provide about 10-20% increase in flexural tensile strengths for large armor. units. The cost increase for the fiber-reinforced concrete equates to an equivalent percent increase in strength.
Existing concrete armor units are described in the U.S. Army Corps of Engineers design manuals Coastal Engineering Manual and the Shore Protection Manual. See, e.g., http://chl.erdc.usace.army.mil/chi.aspx?p=s&a=ARTICLES;104). Popular armor units include dolos, tribar, CORE-LOC®, ACCROPODE®, tetrapod, SAMOA STONE™, Antifer cube, concrete cube, shed and cob. The CORE-LOC® armor unit is protected by U.S. Pat. Nos. 5,441,362 and 5,620,280, each to Melby et al., and each incorporated herein by reference.
Commonly used concrete armor units have features that are advantageous for certain applications but are not suitable for other applications. Advantages include:                Highly porous so good wave dissipation (CORE-LOC®, ACCROPODE®, tetrapod, dolos, random cube, shed, cob, tribar)        Good interlocking in all directions (dolos, CORE-LOC®, tetrapod, Samoa Stone™)        Good structural capacity (small to medium sized CORE-LOC® and ACCROPODE®, tetrapod, cube, Samoa Stone™)        Armor layer can deform without catastrophic failure (random cube)        Simple mould construction (cube)        Simple armor unit to cast (cube, Samoa Stone™)        Simple armor unit to place (cube, Samoa Stone™)Disadvantages include:        Poor interlocking (cube, tetrapod, other armor if not correctly placed)        Marginal structural capacity (very large CORE-LOC® and ACCROPODE®, large tribar, dolos, shed, cob)        Structure is brittle and fails abruptly (most concrete armor layers)        Relatively complex mould construction (CORE-LOC®, tribar, dolos, shed, cob, ACCROPODE®)        Relatively difficult armor unit to cast (CORE-LOC®, tribar, dolos, shed, cob, ACCROPODE®)        Difficult armor unit to place in low visibility or moderate background waves (CORE-LOC®, tribar, dolos, shed, cob, ACCROPODE®)        Relatively large under layer required because pore spaces are large (CORE-LOC®, tribar, dolos, ACCROPODE®)        
For most armor units, it is difficult to achieve adequate interlocking when placing underwater. This is particularly true when the visibility is low and there are background waves during construction. For pattern-placed armor, it is virtually impossible to place them correctly with no visibility or when background waves are present. This condition is quite common. Many armor units require a relatively smooth under layer (CORE-LOC®, ACCROPODE®, tribar, shed, cob, cubes if pattern-placed). Achieving interlocking and a smooth under layer when there is low visibility and background waves is extremely difficult and the uncertainty has led to cost overruns and even breakwater failures.
Relatively slender armor units, such as dolos, CORE-LOC®, ACCROPODE®, tribar, and hollow blocks like the shed and cob, require high-cost moulds and are challenging to cast. Metal mould cost depends on the number of plates and complexity of the bends. Some armor unit moulds require 75-100 plates. Cubes require the fewest plates but have all the concrete concentrated in one mass. This produces high heat of hydration and potential thermal cracking. Tall moulds used for large dolos, CORE-LOC®, ACCROPODE®, and hollow blocks also have potential for significant strength variations throughout the armor unit because the aggregate settles, compaction is greater at the bottom of the mould, and water rises when the concrete is vibrated during casting. High water-to-cement ratios and over-vibration, which can occur in poorly supervised construction, results in degraded armor units. For example, aggregate can concentrate in the lower portion of the unit while the upper portion has an abnormally high water-to-cement ratio yielding weaker concrete. In addition, complex shapes have horizontal or shallow sloping surfaces where water can pool in the mould, further reducing strength. The result is that tall complex shapes depend greatly on the quality of construction processes and can yield less than optimum strength.
The application dictates the appropriate armor unit. For shallow, clear water with insignificant background wave conditions, and waves under eight meters in height, most of the previously discussed armor units can be constructed and placed without difficulty. In these cases, an engineer chooses the least expensive unit that provides the prescribed reliability. However, for low visibility, high background wave conditions, or waves of eight meters or greater, the disadvantages of inexpensive existing armor units mean that construction of a quality structure is going to be difficult and expensive and may even be filled with uncertainty. Further, long slopes in armored configurations provide more opportunity for down-slope settlement and potential armor breakage or displacement as the interlocking is lost. Although cube armor units are relatively easy to construct, they do not interlock so maintenance costs are much higher than other designs and cube armor requires far more concrete than many other designs.
Interlocking concrete armor units or erosion prevention modules are well-known in the patented prior art as evidenced by the Kaneko et al., U.S. Pat. No. 3,614,866 and Chevallier, U.S. Pat. No. 4,347,017.
The Kaneko et al. patent discloses a polypod block comprised of at least three integral pillar-shaped parts joined in an alternately crossed relationship. Hence, the block has at least six appendages which interlock with other blocks so that a large number of blocks can be arranged to form tightly assembled combinations. The pillar-shaped members are joined together with a minimum amount of shared surface area yielding significant stress-concentrations at joints. Because of this, the configuration has a high probability of breaking individual units, potentially leading to massive failure of the configuration. Further, the appendages do not stay connected because the pillar-shaped members have a square cross-section that provides a limited area of frictional engagement with neighboring blocks. Because of the regular arrangement of the individual units, catastrophic failure of the installed configuration can result from the failure of a relatively few armor units. Finally, regularly placed units of the configuration produce an armor layer with very low porosity, providing little wave energy dissipation and therefore little contribution to reduction in wave energy for the protected area in the lee of the configuration.
The Chevallier patent discloses an armor unit, commonly known as the ACCROPODE®, for protecting riverside structures and shorelines. The unit comprises a cubic central core having top and bottom surfaces provided with anvil-shaped legs and opposed front and rear legs in the form of four-sided truncated pyramids. Hydraulic stability characteristics of the Chevallier unit are good if the units are well interlocked but marginal if not because of the anvil-shaped legs that provide minimal unit-to-unit wedging. The units rely primarily on gravity forces from overlying units to enhance individual unit stability. Therefore these units must be placed on steep slopes to assure stability. However, steep-sloped structures have a tendency to fail catastrophically and have proven to have a high probability of failure and risk when used in low-visibility waters, in deep water, or when construction is done in relatively high wave environments. Placed on slope, a Chevallier unit exhibits characteristics of a low-porosity armor layer due to the fact that it is placed in a single layer. This provides less reduction in wave energy than found in an armor layer composed of multiple layers of more slender armor units. Further, the Chevallier blocks require fairly severe constraints and specifications for placement on the breakwater in order to develop enhanced hydraulic stability.
Practical difficulties result in the manufacture, storage and transport of armor units. For example, some armor units have shapes which are not easily cast or formed. For example, the units of Yang, U.S. Pat. No. 6,666,620 B2 contain complex leg formations (“a cylindrical body, tapered lump legs projecting from the cylindrical body and branch legs cylindrically projected through a circular base disposed on the cylindrical body”). Some armor units, such as the Yang units, do not allow for nested placement in yard areas or in shipping barges, and consequently are difficult to store and ship efficiently. Also, some structures are not repairable by simple addition of replacement armor units, but must be partially disassembled. For example, the units of Detiveaux, U.S. Pat. No. 6,361,247 B1 require insertion of a long spike section into the earth and stabilization of the section with guide wires. The amount of wave dissipation is minimal provided through small windows in the top of the structure.
The designs of Melby et al. addressed many inherent weaknesses of the above designs but provided little improvement over existing shapes in fabrication, requiring skilled supervision in casting the final product, as well as for emplacement of individual units where poor visibility or moderate to severe wave action exists.
Concrete armor units are shaped to provide improved performance over stone as armor. The unit's shape may include appendages to promote interlocking between neighboring units. Hundreds of shapes have been developed, however, relatively few shapes are used as noted in the abbreviated list above. Many shapes are either too slender, thus prone to breaking, or provide resistance to wave forces in only a single plane, e.g., Detiveaux.
There is thus a need for a durable interlocking armor unit capable of random placement resulting in a stable configuration that has strong individual units while being relatively straightforward to fabricate. Each unit should have slender appendages to provide improved stability and wave energy dissipation yet be strong enough to prevent failure of any single unit. The unit should be suitable for repair of existing slopes. It should be relatively simple to fabricate and lend itself to ready stacking for storage and shipping, thus reducing overall cost, as well as to emplacement in conditions not conducive to emplacing existing units.